This invention relates to an automatic gain control circuit (which is abbreviated to an AGC circuit hereinafter) and, more particularly, to an AGC circuit for use in combination with a pulse noise cancelling system.
In a radio receiver for an automobile or the like, an input signal is unavoidably accompanied by pulse noises caused by a signal device on a road such as traffic signals and warning lights and an ignition device on an automobile in itself. Audible reproduction of these pulse noises is discomforting to listeners. A pulse noise cancelling system is therefore essential in the radio receiver of the above type.
In a conventional noise pulse cancelling system, the input signal is interrupted during production of the noise pulses to produce an intermittent output signal from which the noise pulses are removed. With this cancelling system however, no output signal is audibly reproduced for a long time in the presence of continuous or long-term noise. The lack of an output signal for such long periods is also disturbing to the listener. As will later be described with reference to two figures of the accompanying drawing, an AGC circuit is used together with the pulse noise cancelling system to remove the above-mentioned problems.
An AGC circuit of this kind is preferably rendered active as slowly as possible in view of the removal of the noise pulses. This is because the active AGC circuit allows the noise pulses to be reproduced to a certain degree. On the other hand, the AGC circuit should be rapidly switched to an inactive state since it will otherwise allow the noise pulses to be reproduced.
With a conventional AGC circuit, it is difficult to attain such requirements. Moreover, the conventional AGC circuit suffers from temperature dependency and inevitably causes click pulses to propagate through any other analog circuits.
It is empirically or experimentally known in the art that, under strong field intensities, listeners accept noises of comparatively high levels. Under weak field intensities, the AGC circuit is preferably rendered active irrespective of noise pulses. It is therefore desirable that the AGC circuit is rendered active when the noise pulses are continuous or high in level and the field intensity are very strong or very weak.